Your feelings are normal and valid. You can't control how you feel. Time, counseling and reflection can clarify and alter them maybe, but your actions are what define you.
You seem very mature in many ways for 26 and you have put so much deep thought into this situation. You are even reaching out for advice even though it is difficult because like so many men you grew up believing "men don't cry" and "real men don't have feelings, let alone share them." This mentality unfortunately stunts so many men that I think they often can't properly name what they are feeling let alone communicate what they need to a partner.

Yet, you ARE still 26. You have been in a committed relationship nearly all of your adult years. Speaking from experience, you can become resentful of people or situations that prevent you from feeling freedom because you have never had much opportunity for that.

This brings me to a concern I have. Everyone here definitely has great points I agree with, but a unique point to add is that unless your GF is dull emotionally, she already knows your feelings. You discussed them already. This brings us back to actions. Like in my circumstance, my husband was shouting what he wanted but never said a word. We rarely can hide from those close to us what we feel entirely
In my circumstance, it was unanswered text messages (even ones needing emotional support), missed opportunities to spend time with me or say something appropriate instead of being silent. It was nights of rolling away from me or falling asleep before I finished my night routine instead of waiting to chat about our day like past nights.
The atmosphere changes. I believe it is more painful to be in a committed relationship and feel alone than to actually be alone.
That is stress. It can equal the stress of a revelation such as your decision to only commit one more year full time to her.
The other complication is you mentioned you live in a small town and are feeling obligated to marry now. That isn't going to get easier to dodge and will keep coming up. Not that their opinion matters, but it puts stress on both of you.
I absolutely commend you for committing your lifetime to caring for your child and for your vow to help the mother always. So many dads "fade in the wash". That is so damaging to a person's psyche and puts them at a disadvantage in knowing how to have relationships with people of both genders.

Your baby's organs and systems are in place. Your baby won't be noticeably affected by mom's stress now, but brain and nervous system development continues for many years now honestly. Mom's stress when the baby is 3 months old will still affect baby in breastmilk and her emotions. Especially if she has debilitating postpartum depression as I did.
Babies change so much their first year of life that every month they look and act so different. You may find you can't leave any more then than now.

I am not saying to leave while she is pregnant, but that later isn't necessarily better or going to work out. I am just bringing up considerations.
In some ways GF is starting the honeymoon phase of pregnancy where she can relax that baby's chances have improved, nausea usually improves, exhaustion improves for a while, and she will feel baby soon. As much as she wants baby, the first 3 to 6 months after birth are so exhausting that this is probably an equally happy time now. It is hard to feel alone and down when you feel flutters and rolls of a miracle baby inside you.